AU-B-37775/89 (618377), the Australian national phase of PCT/AU-89/00267 (WO-89/12510), The Minister for Agricultural and Rural Affairs of the State of New South Wales, discloses a controller for agricultural sprayers where sensors measure the irradiance and radiance (or irradiance and reflectance) of a target area in two bands (eg. red and near infra-red) of the electromagnetic spectrum. The measurements are used to control the spray. Control involves a determination of the relationship between the ratios of the radiance (or reflectance) to the irradiance in each band respectively. The major flaw in this system is that it does not cope with changing light conditions or partly shaded areas in the viewing area. Further it does not provide a size selection function. The plant or weed size at which the controller acts is not able to be adjusted.
Colour analysis is the basis of a variety of discrimination systems operating in a range of circumstances. Examples are seen in U.S. Pat. No. 4,653,014 (Omron) and U.S. Pat. No. 4,797,738 (Tohken). These operate with video signals, operating on components therein to establish the existence of a target condition. In Omron there is seen a totally digital system which uses the R/S, G/S, and B/S signals (where S=R+G+B and R, G, and B are the red, green and blue components of the video signal). This system defines specific colour by analyzing its three signals with reference to upper and lower limits. In Tohken the signals Y (luminance), R-Y and B-Y are compared each with two limit values and analysis determines specific colour. Neither of these systems enables use with sprays in the field where an area which is predominantly green, a weed or other target plant, is to be found in an area of another colour, usually colours such as brown which return a green component in a camera output.